Field of the Invention
The present invention relates to a printed circuit board improved in adhesion between copper circuit layer and resin layer, a method for making such a board, and an apparatus used for making such a board.
Hitherto, for bonding resin to metal, it has been known to form an oxide layer on the surface of metal for improving adhesion as disclosed in "Plating and Surface Finishing", Vol. 69 No 6, pp 96-pp 99 (Jun. 1982) because direct bonding to a smooth surface of a metal does not afford sufficient adhesion. However, many of the metal oxides generally have the problem that upon contact with an acidic aqueous solution, the oxide readily hydrolyzes and dissolves as a metal ion.
Various methods have been studied for surface treatment of copper for the purpose of bonding together copper and resin. It is difficult to obtain sufficient adhesion by bonding a resin to the surface of copper without any such treatment. Therefore, for improving adhesion, formation of an oxide layer of cuprous oxide, cupric oxide, or the like on the surface of copper by various methods has been studied as method for surface treatment of copper. For example, it has been known to treat the surface of copper to which resin is to be bonded, with an alkaline aqueous solution containing potassium persulfate or sodium chlorite to form an oxide layer. Such a method for forming a copper oxide layer is very effective for improvement of adhesion. Generally oxides of copper are readily dissolved by hydrolysis upon contact with acid and so contact with acid must be avoided during the period from treatment to bonding. Furthermore, even after bonding, if acid treatment is conducted, any exposed portion of the oxide layer such as at bonding interface which is exposed at end portion of the cut surface, or at a hole pierced through the bonded surface, dissolves as acid penetrates into interface. As a result the oxide layer of bonding interface is lost. This phenomenon is not preferred for bonding. This phenomenon is a serious problem in laminating and bonding processes for making a multi-layer printed wiring board because they include a step of making through-holes which pierce through bonding interface and step of acid treatment for plating of through-holes.
For solving the above problems, it has been proposed in Japanese Patent Kokai No. 61-176192 to form a film that is resistive to acids by forming a copper oxide layer on the surface of copper and thereafter chemically reducing the oxide layer.
According to the method proposed above, the bonding interface between surface of copper and epoxy resin or polyimide resin is excellent in acid resistance and bonding strength, but mechanical strength at high temperatures is not sufficient and thus this method is still unsatisfactory for printed circuit boards which require high reliability.